1. Technical Field
This invention relates to the fabrication of masks suitable for lithographic processing.
2. Background Art
Projection electron beam lithography systems such as SCALPEL™ utilize a mask to pattern substrates coated with energy sensitive materials, i.e. resists. Two types of scattering masks are stencil masks and continuous membrane masks. In a scattering stencil mask, openings are provided therein. The openings permit the passage of radiation therethrough. Scattering stencil masks are described in U.S. Pat. Nos. 5,567,551, 5,968,336 and 6,100,331. A method of fabricating a scattering stencil mask is also described in Japanese Patent Publication No. 4-240719. As described in the above, the stencil mask pattern is formed by forming a pattern in the membrane.
In continuous membrane scattering masks, a patterned scattering layer is formed on the membrane. Thus, in a continuous membrane mask, the membrane itself is not patterned. The radiation that passes through the membrane is not scattered significantly. The patterned region that is formed on the continuous membrane scatters the radiation more significantly. The scattering contrast between the insignificantly scattering membrane region and the pattern region in conjunction with an aperture located in the back focal plane of the projection optics of the electron projection exposure tool forms the basis for the image. Continuous membrane scattering masks are described in U.S. Pat. Nos. 6,051,346, 5,798,194, and Liddle et al, “Mask Fabrication for Projection electron-beam Lithography Incorporating the SCALPEL Technique,” J. Vac. Sci. 9:3000-3004 (November 1991).
Of these two types of masks, the continuous membrane mask is found to provide good mechanical behavior, desirable electron scattering properties and desirable thermal properties that make it a suitable choice for projection electron beam lithography systems. In addition, because the scattering layer is patterned but the membrane that supports the scattering layer is not, the patterning limitations (e.g. the lithographic limitations associated with patterning features with small dimensions and transferring those features into underlying layers) that are associated with stencil mask fabrication are not presented by the fabrication of continuous membrane masks.
There are however advantages gained by using a stencil mask, such as increased contrast. Prior art processing has described methods used to specifically fabricate masks of either a stencil or a continuous membrane mask.